Seeds are a non-solid block, and they are used to plant Crops on plowed soil / tilled land (see Plow).
Seeds can be obtained by processing their crops in the Processor. Currently every crop can be processed into 2 according seeds each.
Most crops can be found randomly scouring the lands, such as Lettuce and Turnips in most Forests, Turnips also often in Swamplands, on Grass or Dead Grass, as well as Horned Melons on Jungle floors, meaning Detritus. Wheat Seeds can be obtained randomly by collecting Tallgrass or Savannah Tallgrass (even after you've placed it yourself).
Very rarely additionals seed will be found while harvesting. If you harvest your crops too soon (like when they're still sprouts), you will only get a seed each, but no crops.
All crops will need Water, Bog Water or Mineral Water 1 block close to their block to grow. Seeds of Crops can be fertilized with Pigsy Droppings to grow faster (Saplings cannot). Still seeds will stay fallow in too cold (snowy, icy), too high (above ca. 150) or too hot (lava layer) environments as well as in Swamplands.
If a player takes the soiled land under a seed, the seed will fallow but the seed will still be there floating in mid-air (in comparison to the non-solid blocks of red mushrooms or tall grass)
Currently (R30) seeds can be planted only in soiled land, but if a player is quick enough, a seed can be planted on a random solid block, and the result is a fellow seed that can be used as a method to maintain liquid tar in midair without burning it